Pneumatic rubber tires are conventionally prepared with a rubber tread which can be a blend of various rubbers which are, typically, sulfur-curable or sulfur-cured as the case may be, diene-based elastomers.
Sometimes the treads of rubber tires are prepared of a cap/base construction in which the outer portion of the tread is the cap and the underlying part of the tread between the tread cap and the supporting tire carcass is its base. The cap portion is usually designed to be ground-contacting and, thus, has associated properties and the base portion generally overlays the tire carcass and is usually designed to support the cap, therefore, not to be ground-contacting. Such cap/base constructions are well known to those skilled in the art.
The rubber compositions for most components of a tire are typically reinforced with a substantial amount of carbon black reinforcing filler and thereby have a relatively low electrical resistivity for dissipating static electricity to the road from a moving vehicle which utilize such tires. It is believed that such phenomenon is well known to those having skill in such art.
However, it is sometimes desired to provide a tread of a rubber composition which contains a substantial amount of relatively non-electrically conductive reinforcing filler, such as for example precipitated silica, and thereby only a minor amount of carbon black reinforcement, if any, in an amount, for example, of less than 20 phr. Such a tread has a relatively high electrical resistivity because of the low level of carbon black and, thus, has a resistance to dissipating vehicular generated static electricity through the tire to the road. In practice, the relatively high electrical resistivity rubber composition may be the unitary tread, the tread cap and/or tread base. It is believed that such phenomenon is well known to those having skill in such art.
Various methods of providing paths of reduced electrical resistance to the outer surface of a tread cap have been suggested. For example, see U.S. Pat. Nos. 1,797,545, 2,267,503 and 2,641,294; and the following patent publications: French 1,198,271, 1,251,273, 1,279,913 and 1,546,488; German DE 44 17914; Swiss 597,008 and Japanese 57-194,109 and 1-293,208.
Accordingly, for a tire tread construction where unitary tread composition, or tread cap and/or tread base rubber composition has a relatively high electrical resistivity, particularly where it contains only up to about 20 phr of carbon black, it is desirable to provide a suitable path of relatively low electrical resistance from the outer surface of the tire tread to the inner surface of the tire tread, or in a case of a tire of cap/base construction, to the inner surface of the tread cap or tread base, as the case may be.
As used herein, the terms "substantially, or quantitatively, reinforced with reinforcing fillers which are electrically relatively non-conductive such as, for example, silica", and the like are generally used in conjunction with a tire tread, or tread cap and/or tread base in the case of tread of cap/base construction, which contains about 30 to about 100, sometimes preferably about 30 to about 90 phr, of electrically non-conductive filler, and only a minimal amount, if any, of carbon black in which the carbon black is present in not more than about 20 phr. Sometimes, the ratio of non-conductive filler to carbon black may be at least 2/1 and sometimes even at least 10/1.
By the term "carbon black reinforced", it is meant that the rubber components of the tire carcass rubber which are carbon black reinforced, contain a quantitative amount of carbon black reinforcement, normally at least 25 phr, and a minimal amount, if any, of relatively non-electrically conductive reinforcing filler such as, for example, silica. The weight ratio of carbon black to such non-conducting filler might be, for example, at least 5/1.
In the description of this invention, treads of unitary rubber composition and treads of a cap/base rubber compositions are presented. Usually the tread cap rubber composition differs from the tread base rubber composition, particularly since the tread cap is intended to be road-contacting and requires a rubber composition of high abrasion durability. This would be understood by one having skill in such art. Tread wings are often used in a tread construction which extend to and usually over a portion of a tire sidewall. In the description and claims of this invention, while it is not intended to exclude such tread wings, the description of a unitary tread rubber composition and tread cap/base rubber compositions is generally directed to the portion of the tread other than the tread wings, with an understanding that a tread wing rubber composition may be the same or different than that of the unitary tread rubber or the tread cap or base rubber composition.
A footprint of a tire tread is that portion of the tread which is intended to contact the ground when in use. Such footprint, insofar as the description of this invention is concerned, refers to the tire tread whether or not the tire is mounted on a rim or on a vehicle under loaded conditions.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber". In the description herein, rubber and elastomer are used interchangeably.
In the description herein, the term "vulcanized" or "vulcanizable", may, on occasion, be used interchangeably with the terms "cured" and "curable".